1. Field of the Invention
This invention relates to semiconductor laser devices and, more particularly, to a semiconductor laser device of an SBR (Selected Buried Ridge stripe) structure used for digital video discs, magnet-optical discs, laser printers, and the like.
2. Description of the Prior Art
A conventional semiconductor laser device 1 of this kind, as shown in FIG. 3, includes a substrate. On the substrate 2 are formed a buffer layer 3, a lower cladding layer 4a, an active layer 5, a first upper cladding layer 4b and an etch stop layer 6, in this order. A ridge 8a is formed on the etch stop layer 6 by conducting etching after crystal growth. This ridge 8a includes a second cladding layer 4c and an intermediate contact layer 7a. A current restricting layer 8b is formed on the etch stop layer 6 in a manner clamping the ridge 8a from both sides. A GaAs contact layer 7b is formed on the ridge 8a as well as current restricting layer 8b. Further, electrodes 9a and 9b are formed respectively on an underside of the substrate 2 and on a top surface of the GaAs contact layer 7b. 
In an attempt to make this semiconductor laser device 1 compatible with an increase in output power, if the light in the active layer 5 is allowed to partly outdiffuse, the light density within the active layer 5 will be decreased. In such a case, if the outdiffused light from the active layer 5 reaches the optical-absorbent GaAs contact layer 7b and is absorbed by the same layer, the semiconductor laser device 1 possibly deteriorates in characteristics. Due to this, it has been a conventional practice to increase the wall thickness of the ridge 8a, thereby increasing a distance L between the active layer 5 and the GaAs contact layer 7b and hence preventing the light from arriving at the GaAs contact layer 7b. 
In the prior art, the deterioration in laser characteristic resulting from output power increase has been prevented by thus increasing the wall thickness of the ridge 8a. This, however, has resulted in an increase in side etch amount on the ridge 8a upon implementing an etching process for providing the ridge 8a, resulting in narrowed top end width (current passage width) of the ridge 8a. Thus, there has been a problem that the device is increased in series resistance leading to an increase in operating voltage.
Therefore, it is a primary object of the present invention to provide a novel semiconductor laser device.
It is another object of the present invention to provide a semiconductor laser device having an SBR structure which can prevent against degradation in laser characteristic due to increase of output without incurring increase in device series resistance.
A semiconductor laser device according to the present invention, comprises: a substrate; a lower cladding layer, an active layer and a first upper cladding layer which are formed in order on the substrate; a ridge formed on the first upper cladding layer, the ridge including a second upper cladding layer; a current restricting layer formed in a manner clamping the ridge; a contact layer formed on the ridge and the current restricting layer; and a third upper cladding layer formed between the ridge and current restricting layer and said contact layer.
The distance of from the active layer to the contact layer is secured sufficiently long by thus forming the third upper cladding layer between the ridge and current restricting layer and the contact layer. Consequently, there is no fear that the outdiffused light from the active layer arrive at the contact layer.
According to the present invention, because there is no need to increase the thickness of the ridge as conventionally done, it is possible to prevent the width of a ridge top end from being narrowed due to side over-etch on the ridge. Thus, the deterioration in laser characteristic due to output power increase can be prevented without incurring increase in series resistance.
The above described objects and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.